Antibodies are an important immune mechanism for clearing most viruses and long-lived antibodies form the functional basis of all currently used vaccines. Long-lived anti-viral antibody responses are generated via interactions between virus-specific B cells and virus-specific T follicular helper (Tfh) cells, which are a novel subset of CD4+ T cells that preferentially express CD40 ligand and IL-21 - factors that promote B cell proliferation and differentiation. Although the production of long-lived antibody responses requires help from Tfh cells, the factors that control Tfh differentiation are poorly understood. Our preliminary data demonstrate that high levels of IL-2 inhibit the differentiation of Tfh cells. Therefore, we hypothesize that the mechanisms that control IL-2 production, IL-2 signaling and IL-2 availability will also control the differentiation of Tfh cells and either promote or prevent anti-viral antibody responses. The experiments in Project 1 will determine how the persistence of viral antigen, the availability and type of costimulation, and the production and availability of IL-2 regulate T follicular helper differentiation and thus control long-lived antibody responses. Experiments in Aim 1 will test whether different types of virus infection (chronic-systemic, acute-systemic or acute-local) alter TCR signaling, IL-2 receptor expression and IL-2 production and thus, lead to changes in Tfh differentiation and antibody responses. Experiments in Aim 2 will test whether changes in costimulation or the expression of costimulatory receptors alter IL-2 production and Tfh differentiation. Experiments in Aim 3 will test whether external factors, such as IL-2 consumption by Tregs, alter IL-2 availability and control Tfh differentiation and antibody production.